1. Field of the Invention
The present invention relates to a composition that is photopolymerizable upon absorption of light, the composition including a binder, a polymerizable compound, a sensitizer, and a photoinitiator.
The present invention also relates to a photopolymer printing plate precursor including the photopolymerizable composition, and relates to a method of making a lithographic printing plate therewith.
2. Description of the Related Art
Compositions that are photopolymerizable on absorption of light are well known in the art. Such compositions usually include a binder, a polymerizable compound, a sensitizer, and a photoinitiator and are disclosed in, for example, EP 1 349 006 A. From this document it is also known, that radical chain transfer agents, as described in EP 107 792, can be used to achieve a high sensitivity and that such radical chain transfer agents preferably are sulfur compounds like the mono-thiols 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, or 2-mercaptobenzimidazole.
As there is an ongoing trend to expose the photopolymerizable compositions with low power light sources like laser-LEDs, the sensitivity of the known compositions including mono-thiols is still unsatisfactory. Moreover, the optimization of photopolymerizable compositions and of printing plate precursors often is accompanied by a loss in sensitivity. This is, for example, the case when stabilizing the composition against environmental effects or when limiting the absorption spectra to allow for room light handling. Therefore, it is necessary to compensate for such a sensitivity loss.
Specific aliphatic polythiols for radiation curable compositions are known from, for example, U.S. Pat. No. 4,120,721, wherein the composition forms a solid cured polythioether on exposure, but does not disclose a sensitizer plus photoinitiator system in combination with specific polythiols. The same is true for U.S. Pat. No. 3,993,549, wherein the composition is based on a particulate urea; FR 2,227,558; U.S. Pat. No. 3,753,720; U.S. Pat. No. 3,676,195; and U.S. Pat. No. 3,661,744.
Aliphatic thiols that have a branched structure in α- and/or β-position of the mercapto group are disclosed in WO 03/72614 and JP 2003-252918 A2 to be suitable for a high sensitivity of photosensitive compositions, wherein JP 2003-252918 A2 discloses such compounds in combination with organoboron complexes.
The use of organoboron complexes is also taught in EP 1 031 579, wherein such boron complexes are combined with a thiol-group-containing compound, that preferably is selected from 5 specific compounds with one, three, or four thiol groups.
The polythiols are usually disclosed in the prior art to take part in the photocuring process and to react in combination with electron rich or electron poor polyenes by the thiol-ene mechanism. Therefore, they are commonly used in similar stoichiometric amounts as the polyenes and selected in view of the solid polythioether products that are formed by this reaction. Although the invention underlying, for example, WO 03/72614 is said to achieve a high sensitivity, it is disclosed on page 30 of this document that a super-high pressure mercury lamp, metal halide lamp, xenon lamp and the like is generally used as the light source and the example compositions only have a sensitivity of no higher than 10 mJ/cm2. This demonstrates the unsatisfactory sensitivity of the compositions of the prior art. As for low power light sources, the sensitivity has to be significantly higher than 1 mJ/cm2 and, therefore, has to be in the μJ/cm2 region (a lower numerical value corresponds to a higher sensitivity, see below).
Therefore, there still is a need to find further thiol compounds and a photopolymerizable composition optimized for such compounds, that results in a very high sensitivity, because a broader range of compounds gives a better chance to optimize the photopolymerizable composition, in particular, those including a binder, a polymerizable compound, a sensitizer, and a photoinitiator.
Optimization with respect to the properties and the costs as well is particularly important when using the photopolymerizable composition for the preparation of a lithographic printing plate precursor.
In addition, the known photopolymerizable compositions are still unsatisfactory in that they are not resistant enough after exposure and processing. In particular, the problem that known photopolymerizable compositions are not hard enough results, for example, when they are used in a lithographic printing plate precursor for a printing plate made from such a precursor that has a short printing lifetime. The printing lifetime is also called run-length on the press.
In lithographic printing, a so-called printing master, such as a printing plate, is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a printed copy is obtained by applying ink to the image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e., ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e., water-accepting, ink-repelling) areas. In so-called “driographic” printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film (CtF) method, wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout, and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called a plate precursor and after plate processing, a printing plate is obtained which can be used as a master. Since about 1995, the so-called ‘computer-to-plate’ (CtP) method has gained a lot of interest. This method, also called ‘direct-to-plate’, bypasses the creation of film because the digital document is transferred directly to a printing plate precursor by means of a so-called plate setter. A printing plate precursor for CtP is often called a digital plate.
Digital plates can roughly be divided into three categories: (i) silver plates, which work according to the silver salt diffusion transfer mechanism; (ii) photopolymer plates which contain a photopolymerizable composition that hardens upon exposure to light; and (iii) thermal plates of which the imaging mechanism is triggered by heat or by light-to-heat conversion. Thermal plates are mainly sensitized for infrared lasers emitting at 830 nm or 1064 nm. Typical photopolymer plates are sensitized for visible light, mainly for exposure by an Ar laser (488 nm) or a FD-YAG laser (532 nm). The wide-scale availability of low cost blue or violet laser diodes, originally developed for data storage by means of DVD, has enabled the production of plate setters operating at a shorter wavelength. More specifically, semiconductor lasers emitting from 350 nm to 450 nm have been realized using an InGaN material.
Radicals are involved in the hardening reaction of the photopolymerizable composition of photopolymer plates and the hardening reaction is known to be adversely affected by oxygen. To reduce this problem, it is known to provide the photosensitive coating with a protective coating, also called an oxygen barrier layer, protective overcoat, or overcoat layer.
After imaging (exposing) the photopolymer printing plate precursor, the plate is heated for a short time to high temperatures before the overcoat is washed off and the photolayer is developed. This heating step is hereinafter called a pre-heat step. During the pre-heat step, typical temperatures, when measured at the back of the plate, from about 90° C. to about 150° C. are used for a short time, typically between about 10 seconds and about 1 minute. As the conditions of the pre-heat step vary with different types of processors and even for the same processor, a printing plate should exhibit consistent results irrespective of the pre-heat conditions, in particular the temperature. The range of pre-heat conditions, wherein a printing plate exhibits consistent results, is called the pre-heat latitude of the plate.